The present invention finds application particularly in third-generation radio networks such as Wideband Code division Multiple Access (WCDMA) or Long Term Evolution (LTE) type of networks. However, the techniques may be applicable also in connection with other types of radio networks, such as GSM, CDMA etc, and in general in any type of network employing a layered cell structure.
In most Heterogeneous Network (HetNet) scenarios using a layered cell structure, one or more low power nodes, such as Pico cells, are placed throughout a macro-cell layout to increase capacity in hotspots.
A Base Band Unit (BBU) pool is a good solution to realize a layered HetNet architecture. Employing a BBU pool, all baseband processing of a geographical area are centralized into one remote Main Unit (MU), e.g. in the Radio Network Control (RNC) office. Remote Radio Units (RRU) are installed near to antennas and connected directly to the MU by Optical Interface Link (OIL) cables to transport IQ data to the MU. IQ data represents the phase and amplitude of a signal received by the RRU. RRUs can have high output power to create Macro cells or utilizing lower power to create Pico cells, depending on the capacity requirement in the area. Macro cells are cells utilizing a higher output power and are thus covering a larger geographical area, whereas Pico cells are using lower output power and are thus covering smaller geographical areas, most commonly at hotspots with intense traffic. IQ data from all kinds of RRUs are processed in the centralized MU.
FIG. 1 shows the architecture of BBU pool HetNet, where the centralized BBU in the MU processes baseband signals for all the RRUs connected to it. Baseband processing in a centralized BBU also include, among many other things:                Control channel signalling generation for each RRU cell        UE information management for each RRU        UE scheduling for each RRU cell        Joint radio resource coordination among RRUs        
Pico RRU has the advantage of being small, flexible and easy to install, as well as providing an economic solution for increasing capacity. However, the power supply systems, especially big and expensive backup batteries usually restrict the installation convenience and reduces the advantages of Pico RRUs. Backup batteries often require a suitable place to be installed providing the right temperature and humidity, thus restricting the flexibility of Pico cell site selection.
In many implementations, in order to reduce cost and footprint, small volume backup batteries are chosen. The small volume backup batteries only provide a few minutes of power to be able to handover traffic to other cells without dropping calls. The connected calls will drop if there is no battery backup.
Consequently, to improve cell site selection flexibility for pico cells, and to be able to reduce Capital expenditure (CAPEX) and Operational expenditure (OPEX) of Pico cells, there is need for a fault tolerance solution without battery backup system.